From alumina powder and TEOS, $alpha$-Al2O3/SiO2 composite powder for reaction-sintered mullite was synthesized by heterogeneous coagulation and surface coating, and investigated the mullitization reaction and sintering behavor of these powders. In $alpha$-Al2O3/SiO2 composite powder prepared by heterogeneous coagulation, each alumina particles were surrounded by silica particles of 50~60 nm in size. And the alumina particles in composite powder prepared by surface coating were coated by uniform silica layer with thickness of 50 nm. In both methods, mullitization reaction was completed at 1$650^{circ}C$ for 3h, and specimen sintered above 145$0^{circ}C$ was about 95% fo the theoretical relative density. Mullite grains formed from the reaction with composite powders showed spherical shape with a size of 1~2${mu}{ extrm}{m}$.